1,720,980 research outputs found
A Numerical Investigation on the Potentials of Water Injection to Increase Knock Resistance and Reduce Fuel Consumption in Highly Downsized GDI Engines
Full text linkAbstract3D CFD analyses are used to analyse the effects of port-injection of water in a high performance turbocharged GDI engine. Particularly, water injection is adopted to replace mixture enrichment while preserving, if not improving, indicated mean effective pressure and knock resistance. A full-load / maximum power engine operation of a currently made turbocharged GDI engine is investigated comparing the actual adopted fuel-only rich mixture to stoichiometric-to-lean mixtures, for which water is added in the intake port under constant charge cooling in the combustion chamber. In order to find the optimum fuel/water balance, preliminary analyses are carried out using a chemical reactor to evaluate the effects of charge dilution and mixture modification on both autoignition delays and laminar flame speeds. Thanks to the lower chemical reactivity of the diluted end gases, the water-injected engine allows the spark advance (SA) to be increased; as a consequence, engine power target is met, or even crossed, with a simultaneous relevant reduction of fuel consumption
Understanding the origin of cycle-to-cycle variation using large-eddy simulation: Similarities and differences between a homogeneous low-revving speed research engine and a production DI turbocharged engine
Full text linkA numerical study using large-eddy simulations (LES) to reproduce and understand sources of cycle-to-cycle variation (CCV) in spark-initiated internal combustion engines (ICEs) is presented. Two relevantly different spark-ignition (SI) units, that is, a homogeneous-charge slow-speed singlecylinder research unit (the transparent combustion chamber (TCC)-III, Engine 1) and a stratifiedcharge high-revving speed gasoline direct injection (GDI) (Engine 2) one, are analyzed in fired operations. Multiple-cycle simulations are carried out for both engines and LES results well reproduce the experimentally measured combustion CCV. A correlation study is carried out, emphasizing the decisive influence of the early flame period variability (1% of mass fraction burnt (MFB1)) on the entire combustion event in both ICEs. The focus is moved onto the early flame characteristics, and the crucial task to determine the dominant causes of its variability (if any) is undertaken. A two-level analysis is carried out: the influence of global parameters is assessed at first; second, local details in the ignition region are analyzed. A comparison of conditions at combustion onset is carried out and case-specific leading factors for combustion CCV are identified and ranked. Finally, comparative simulations are presented using a simpler flame deposition ignition model: the simulation flaws are evident due to modeling assumptions in the flame/flow interaction at ignition. The relevance of this study is the knowledge extension of turbulence-driven phenomena in ICEs allowed by advanced CFD (Computational Fluid Dynamics) simulations. The application to different engine types proves the soundness of the used models and it confirms that CCV is based on enginespecific factors. Simulations show how CCV originates from the interplay of small- and large-scale factors in Engine 1, due to the lack of coherent flows, whereas in Engine 2 the dominant CCV promoters are local air-to-fuel ratio (AFR) and flow velocity at ignition. This confirms the absence of a generally valid ranking, and it demonstrates the use of LES as a development and designorienting tool for next-generation engines
A RANS-Based CFD Model to Predict the Statistical Occurrence of Knock in Spark-Ignition Engines
No full textEngine knock is emerging as the main limiting factor for modern spark-ignition (SI) engines, facing increasing thermal loads and seeking demanding efficiency targets. To fulfill these requirements, the engine operating point must be moved as close as possible to the onset of abnormal combustion events. The turbulent regime characterizing in-cylinder flows and SI combustion leads to serious fluctuations between consecutive engine cycles. This forces the engine designer to further distance the target condition from its theoretical optimum, in order to prevent abnormal combustion to severely damage the engine components just because of few individual heavy-knocking cycles. A RANS-based model is presented in this study, which is able to predict not only the ensemble average knock occurrence but also a knock probability. This improves the knock tendency characterization, since the mean knock onset alone is a poorly meaningful indication in a stochastic event such as engine knock. The model is based on a look-up table approach from detailed chemistry, coupled with the transport of the variance of both mixture fraction and enthalpy. These perturbations around the ensemble average value are originated by the turbulent time scale. A multivariate cell-based Gaussian-PDF model is proposed for the unburnt mixture, resulting in a statistical distribution for the in-cell reaction rate. An average knock precursor and its variance are independently calculated and transported, and the earliest knock probability is always preceding the ensemble average knock onset, as confirmed by the experimental evidence. This allows to identify not only the regions where the average knock first occurs, but also where the first knock probability is more likely to be encountered. The application of the model to a RANS simulation of a modern turbocharged direct injection (DI) SI engine is presented and a small percentage of knocking cycles is predicted by the model although the average behavior is knock-free, in agreement with the experiments. The estimate of the knocking probability improves the consolidated “average knock” RANS analysis and gives an indication of the statistical knock tendency of the engin
Large-eddy simulation of cycle-resolved knock in a turbocharged SI engine
No full textThe paper presents a numerical study of cycle-to-cycle variability in a turbocharged GDI engine. The Large-Eddy Simulation technique is adopted in this study in conjunction with the recent ISSIM-LES model for spark-ignition, allowing a dedicated treatment of both the flame kernel formation and flame development phases. Numerical results are compared with an extended dataset of experimental test-bed acquisitions, where the engine is operated at knock-limited spark advance. The agreement of both ensemble averaged combustion pressure history and of its standard deviation confirm the validity of the adopted numerical framework able to correctly quantify the degree of CCV measured by the experiments. Knock tendency is evaluated by means of an in-house developed knock model, based on a tabulation technique for AI delays of the same RON98 gasoline as the one used in experiments. The results confirm the knock-free condition of the experimental KLSA, for which the cycle-resolved knock signature is extremely weak just for the cycles in the highest band of the CCV-affected combustion. The visualization of the pressure wave allows to identify the exhaust side as the most knock-prone region. Finally, spark-advance is increased by 3 CA with respect to the experimental edge-of knock limit, in order to simulate an experimentally prevented operating condition. Local pressure measurements mimicking flush-mounted transducers confirm the severe knock damage related to this condition. The predictive capability of the combustion CCV and of the adopted knock model confirm the heavy and recurrent cycle-resolved knock damage
Two-Stage Turbocharging for the Downsizing of SI V-Engines
No full textAbstractOne of the most critical challenges for the specific power increase of turbocharged SI engines is the low end torque, limited by two aspects. First, the big size of the compressor necessary to deliver the maximum airflow does not allow high boost pressures at low speed, due to the surge line proximity. Second, the flame front velocity may become slower than the end gas auto-ignition rate, thus increasing the risk of knocking.This study is based on a current SI GDI V8 turbocharged engine, modeled by means of CFD tools, both 1d and 3d. The goal of the activity is to lower by 20% the displacement, without reducing brake torque, all over the engine speed range.It was decided to adopt a smaller bore, keeping stroke constant. Obviously, the combustion chamber, the valves and the intake-exhaust ports have been re-designed, as well as the whole intake and exhaust system. Instead of the two turbochargers, one for each bank of cylinders, a triple-turbocharger layout has been considered.The development of the engine has been carried out by means of 1D engine cycle simulations, using predictive knock models, calibrated with the support of both experiments and CFD-3d simulations. A few operating conditions for the final configuration have been also analyzed by means of a 3-d CFD tool.The paper presents the results of this activity, and describes in details the guidelines followed for the development of the engine
Combination of G-Equation and Detailed Chemistry: An application to 3D-CFD hydrogen combustion simulations to predict NOx emissions in reciprocating internal combustion engines
No full textIn the recent years, the growing pressure by the European Union to phase out the internal combustion engines has raised the quest for alternative solutions for low-environmental-impact mobility. Nevertheless, concerns on the life-cycle emissions of battery electric vehicles and perplexities on the socio-economic sustainability of the ecological transition suggest that maintaining the interest in internal combustion engines can be strategic, provided that carbon-neutral fuels are adopted. On the basis of the technological neutrality principle, relying on already existing and well-established technologies requires less effort and cost to convert the whole road transport. Moreover, the adoption of bio- or e-fuels obtained from renewable sources widely spread across the globe is not of secondary importance. In fact, cost reduction and worldwide diffusion of the resources are both main promoters of socio-economic sustainability. In this scenario, green hydrogen represents one of the main solutions for the survival of reciprocating engines. Since the production is solely based on renewable energy sources, it is not simply characterized by zero CO2 emissions at the tailpipe, but it can be considered overall carbon neutral. A technical drawback in the use of hydrogen is represented by emissions of nitrogen oxides (NOx), due to the ever-present high temperature combustion process. For this reason, an ad-hoc design is mandatory to minimize NOx production, and CFD can be a valid tool to reduce cost and time to market for the development of hydrogen engines. In this regard, the current work proposes a 3D-CFD numerical methodology, based on the combination of G-Equation and Detailed Chemistry models, for NOx prediction in in-cylinder simulations of reciprocating internal combustion engines fueled with hydrogen. Although the combination of level-set method and chemical kinetics is not a novelty in literature, it is the first time that it is applied to evaluate NOx emissions in H2 engines. The proposed approach is validated against experimental data on a direct injection, spark ignition, hydrogen engine. The methodology is able to properly predict NOx emissions at different mixture qualities, revving speeds and spark times. The total number of investigated cases is 17, which is a large set of simulations compared to the existing literature. Considering the best chemical mechanism (i.e. the one providing the best results among the tested ones), the error in the NOx prediction is always lower than 25% for all the simulations. Once the methodology is validated, the effect of spark and injection timings on NOx is discussed. Such a deepening is useful to emphasize the potential of the CFD to investigate phenomena leading to emission formation and, thus, to optimize engine parameters for NOx reduction
Assessment of Knock Tendency in a Hydrogen-Fuelled High-Performance Internal Combustion Engine: A Chemistry-Based Numerical Study
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Effects on knock intensity and specific fuel consumption of port water/methanol injection in a turbocharged GDI engine: Comparative analysis
No full textThe recent rise in fuel prices, the need both to reduce ground transport-generated emissions (increasingly constrained by legislation) and to improve urban air quality have brought fuel-efficient, low-emissions powertrain technologies at the top of vehicle manufacturers' and policy makers' agenda. To these aims, engine design is now oriented towards the adoption of the so-called downsizing and down-speeding techniques, while preserving the performance target. Therefore, brake mean effective pressure is markedly increasing, leading to increased risks of knock onset and abnormal combustions in last-generation SI engines. To counterbalance the increased risks of pre-ignition, knock or mega-knock, currently made turbocharged SI engines usually operate with high fuel enrichments and delayed (sometimes negative) spark advances. The former is responsible for high fuel consumption levels, while the latter induce an even lower A/F ratio (below 11), to limit the turbine inlet temperature, with huge negative effects on BSFC. Possible solutions to increase knock resistance are investigated in the paper by means of 3D-CFD analyses: water, water/methanol emulsion and methanol are port-fuel injected to replace mixture enrichment while preserving, if not improving, indicated mean effective pressure and knock safety margins. The aim of the work is therefore the replacement of the gasoline-only rich mixture with a global stoichiometric one while avoiding power loss and improving fuel consumption. In order to maintain the same knock tendency, water, methanol or a mixture of the two is then added in the intake port to keep the same charge cooling of the original rich mixture. Different strategies in terms of methanol/water ratios of the port injected mixture are compared in order to find the best trade-off between fuel consumption, performance and knock tendency
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